The present invention relates to an adaptive speech signal detector capable of adaptively setting a threshold value for use in a 4-wire telephone channel.
To increase the efficiency of the transmission of a speech signal over a telephone channel, the realization of a speech signal detector with the excellent detection capability is often desired. For instance, a DSI (digital speech interpolation) system to improve the channel utilization efficiency is a system based on the fact that the speech activity, i.e., the average length of time occupied by the speech of a subscriber on the phone is less than 50 percent of the total length. More particularly, in a given terminal station, the states of the respective channels connected to a large number of subscribers (hereinafter called subscriber channels) are supervised to select out of subscriber channels only those subscriber channels on which speech signals are present, and the transmission is carried out for another terminal station. Since the speech activity of an average subscriber is generally 30-40%, the number of the channels for the transmission between these terminal stations (DSI channels) can be made about one-half in number of the subscriber channels.
In such DSI system, channel interruption frequently occurs during the conversation, and so, the detection capability of a speech detector is one of principal factors to determine the characteristics of the DSI system. For this reason, if the speech detection takes too much time, the time required for performing the speech detection for a subscriber channel on which a speech signal begins to appear and for connecting the subscriber channel to a DSI channel, is increased. Accordingly, the speech on the subscriber channel before it is connected to the DSI channel is not transmitted to a party subscriber with the result that speech-front mutilation occurs frequently. On the other hand, when the speech detection rate is increased to reduce the mutilation, the speech signal detector increasingly malfunctions due to noise, the speech activity is increased, causing a concern that the probability of the saturation of the DSI channels is enlarged to cause the speech interruption.
To solve such problems in the DSI system, the use of an adaptive speech detector in which the threshold value for the speech detection is adaptively varied depending on the noise level on a channel, or a speech detector in which in addition to amplitude information of a speech signal, the zero-crossing information of the speech signal is used, has been proposed. Such a speech signal detector having the threshold value adaptively set depending on the channel noise level is proposed in FIG. 1 of the U.S. Pat. No. 4,028,496 or in FIG. 2 of the U.S. Pat. No. 4,052,568. Furthermore, the speech detector using the zero-crossing information is disclosed in the U.S. Pat. No. 4,001,505.
In such an adaptive speech signal detector, the threshold value is set at a value as small as possible within the range where noise present on a channel is not erroneously detected, and if the magnitude of the noise signal level is gradually varied, the threshold value is also varied following the variation of the noise signal level. As a result, in case where the noise amplitude is small, the threshold value takes a small value so that even low-level speech can be easily detected and the speech mutilation can be reduced. Whereas in the case of the large noise amplitude, the threshold value is increased, and as a result, the detector does not malfunction due to noise of the large amplitude. Although the speech signal detection is delayed due to the increase of the threshold value, the degree of the degradation in quality of speech signals caused by the increase of the speech mutilation is small, if the noise amplitude is large, because the quality of the original speech signals is already degraded. In view of these facts, the adaptive speech signal detector would appear to be a detector suitable for the DSI. However, since the application of DSI system to channels on which echo signals are present, is accompanied with the enhancement of the virtual speech activity due to the echo signals, the DSI efficiency (the degree of the reduction in number of channels by employing the DSI system) is remarkably lowered. The conventional telephone channel composed of a 2-wire channel and a 4-wire channel can not avoid such an echo signal. Accordingly, an echo suppressor is normally interposed between the DSI system and subscribers.
For details of such an echo suppressor, reference is made to an article by P. T. Brady and G. K. Helder entitled "Echo Suppressor Design in Telephone Communications" published in The Bell System Technical Journal, Vol. 42, No. 6, pp. 2893-2917, November issue, 1963. The operation of the echo suppressor will be briefly described hereunder.
An echo suppressor performs switching operations for suppressing an echo signal in such a manner that a large loss is provided or the interruption is made at a transmitter when a received signal is larger than a certain fixed value and is also larger than a transmission signal. For this reason, as soon as a speech signal is given to a receiver, the echo suppressing switch is actuated to prevent the echo signal from being outputted and at the same time, the noise level at the output terminal of the echo suppressor changes momentarily. If the echo suppressor is interposed between subscribers and the DSI system, then in response to the operation of the echo suppressing switch, the noise level on the channel inputted to the transmitter of the DSI system similarly changes immediately. Consequently, the adaptive speech detector used in the DSI system malfunctions at the time point when the noise level has changed abruptly from a small amplitude to the normal one.
Furthermore, in an echo suppressor, when a speech signal of a near end subscriber which is deemed not to be an echo signal is detected at a transmitter, the attenuation of 5 to 6 dB (decibels) is inserted into a receiver in order to relatively intensify (in order to put a preference on) the speech signal of this subscriber against a speech signal of a remote end subscriber. In this case also, there is not only the possibility of the noise level change on the transmitter-side subscriber channel but also the possibility of the malfunction of the adaptive speech signal detector.
Thus, the adaptive speech signal detector cannot be directly used in the DSI system. On the other hand, though the above-mentioned detection system employing the zero-crossing information improves the detection capability for periodic speech signals or signals having a number of high frequency components, there still remains the problem that the detection for signals of a small amplitude and of many low frequency components are difficult.
The conventional adaptive speech signal detector, therefore, has a disadvantage in that its interposition in 4-wire channels between the DSI system and subscribers does not permit the achievement of the expected performance.